Grasp: A New Herbicide for Managing Weeds in Rice

Effectiveness of Grasp at controlling certain weeds at selected rates and timings

Bill Williams

Grasp (penoxsulam) is a triazolopyrimidine sulfonamide herbicide developed by Dow AgroSciences for use in drill- and water-seeded rice. Grasp has excellent foliar activity and some residual control of selected weeds and can be applied at 2.0 to 2.8 ounces per acre from one-leaf rice until 60 days before harvest. The 2005 launch of Grasp by Dow AgroSciences was on limited basis, and label changes are expected for the 2006 season. As always, the LSU AgCenter strongly suggests that a current label be consulted before using any herbicide, including Grasp.

LSU AgCenter weed scientists began working with Grasp in 2003, but it wasn’t until 2004 that in-depth research could be conducted due to limited product availability. The primary goal of this research was to determine what role, if any, Grasp could play in managing weeds in Louisiana rice crops.

In research conducted at the Northeast Research Station, Grasp demonstrated excellent activity on barnyardgrass, hemp sesbania, rice flatsedge, ducksalad, dayflower and purple ammannia in both drill- and water-seeded rice. In off-station tests, Grasp has failed to control smallflower umbrella sedge, indicating that it may not be effective against all annual sedges.

In drill-seeded rice, 2.0 ounces per acre of Grasp controlled small weeds when applied to two- to three-leaf rice, but at least 2.3 ounces per acre were needed at the four- to five-leaf stage for consistent control of larger weeds. In 2005, 2.5 ounces per acre of Grasp controlled both barnyardgrass and sesbania post flood.

In water-seeded rice, Grasp controlled barnyardgrass best when applied from “pegging,” when a tiny root emerges from the seed, through two- to three-leaf rice. Sesbania control was best when Grasp (at 2.3 ounces per acre) was applied at the two- to three-leaf stage. The best control of purple ammannia and ducksalad was observed from treatments at pegging.

Grasp at 2 ounces per acre plus 1.3 pints per acre of Command applied to one- to three-leaf rice has provided excellent control of barnyardgrass, Amazon sprangletop, hemp sesbania and rice flatsedge. In some studies, additional applications were needed to control sesbania coming up after the application. Grasp plus Command must be applied and activated before sprangletop emerges.

Clincher-plus-Grasp combinations control barnyardgrass, sprangletop, flatsedge and sesbania. Overall, weed control was best when Grasp plus Clincher was applied at the two- to three-leaf rice stage. Tank mixing Clincher with Grasp reduced post-flood sprangletop control in 2005 compared to Clincher alone.

Newpath plus Grasp combinations in Clearfield rice were also promising. Texasweed control is improved, and sesbania is controlled when Newpath is tank mixed with Grasp. In 2004, a slight reduction in red rice control was observed when Grasp was tank mixed with the last Newpath application. A reduction in red rice control was not observed in 2005.

Grasp-plus-Londax combinations also were effective at managing Texasweed in conventional rice in 2005. Grasp alone does not control Texasweed.

Alligatorweed is becoming more challenging throughout Louisiana. Grasp at 2.3 ounces per acre controls alligatorweed as well as 11 ounces per acre of Grandstand for about four weeks but is less effective after that. The best alligatorweed control – 90 percent four weeks after treatment and 85 percent six weeks after treatment – was observed from 2.9 ounces of Grasp per acre or 1 pint of Grandstand per acre. Grasp plus Grandstand at lower rates has not been effective at controlling alligatorweed. It is expected that Grasp will be an effective tool for controlling new alligatorweed infestations. However, in fields where alligatorweed has been allowed to establish an extensive root system, Grasp will only provide suppression.

Overall, Grasp compares very well to standard herbicides and has the potential to control some important broadleaf weed problems in rice. At this time Grasp appears to be promising in Clearfield rice, drill-seeded rice and water-seeded rice. At the Northeast Research Station, Grasp has consistently controlled barnyardgrass, hemp sesbania, rice flatsedge, ducksalad and purple ammannia. Grasp also has demonstrated excellent activity on alligatorweed, but it has limited translocation, and regrowth is expected. When tank-mixed with Londax and Newpath (in Clearfield rice), Grasp has demonstrated excellent activity on Texasweed.

A number of questions concerning Grasp need further evaluation. First, a better understanding of rice tolerance at rates higher than 2 ounces per acre when applied at the one- to three-leaf stages is needed. Second, the potential of antagonism issues with Clincher (sprangletop) and Newpath (red rice) needs further evaluation. Additional work with Texasweed and alligatorweed is also needed to confirm 2005 results.